Imidazole derivatives as thermal latent catalyst for thiol-michael reaction thermosetting resins

Abstract

Imidazole is an excellent nucleophilic initiator in thiol-Michael addition reaction of thiol and vinyl sulfone at room temperature. To delay the gelation of the monomer mixture, thermal latent curing agents based on imidazole are designed and the latency and curing efficiency towards thiol-vinyl sulfone system are evaluated. N-carbonylated imidazole derivatives are non-nucleophilic towards Michael reaction with vinyl sulfone. However, during curing at elevated temperature (>80 °C), thiol monomer thiolyze imidazole derivative and the released imidazole acts as a nucleophilic initiator. It is found that the viscosity at room temperature of thiol-vinyl sulfone system with 2b or 2d imidazole derivatives increases slowly up to over 10 h, which permits sufficient time for processing such as mixing, degassing and injection into the mold. In the application of 2b or 2d in network polymerization of polythiol and divinyl sulfone, both catalysts have excellent curing ability with a 98% and 95% thiol conversion, respectively within 7 h at 100 °C. Using 2b or 2d, transparent thiol-vinyl sulfone resins with a refractive index of 1.57 and an Abbe number of 47.78 are successfully produced.